This invention relates to the fabrication of photoreceptors suitable for application to xerographic printers and like machines that use coherent light sources. More particularly, this invention relates to a multilayered photoreceptor having a designated substrate surface roughness that minimizes or eliminates an interference-fringe print defect in the resulting printer output where there is absent an undercoat layer between the substrate surface and a metal oxide layer over the substrate. In the present invention, the metal oxide layer is not considered an undercoat layer.
Xerographic printers and like machines that use multilayered photoreceptors in conjunction with a coherent light source suffer from an interference effect that manifests as a printable defect that can be described as a series of dark and light interference fringes that resemble wood grains. The use of coherent illumination sources in conjunction with multilayered photoreceptors produces the interference effect through the interaction between various reflected components of the incident light whose difference in optical path length varies from one area of the photoreceptor to another. Such spatial variation in the optical path length arises because the coated layers have inherent spatial thickness variations imposed by limitations in the coating process. The spatial variation in the optical path length in turn produces absorption variation in the charge generating layer of the photoreceptor, resulting in the interference-fringe defect in prints generated by these xerographic machines.
FIG. 1 is a schematic view of a typical photoreceptor of a multilayered design. In FIG. 1, the photoreceptor 10 includes a substrate 1, an undercoat layer 2, a charge generating layer 3, and a charge transport layer 4.
In this device, which comprises three organic layers 2-4 coated on a metallic substrate 1, an incident light beam 5 is directed at the charge transport layer 4. The primary light beam 5 is then reflected from the planes that define interfaces 7, 9A, 9B and 9C between the layers 1-4 of the multilayered photoreceptor. More specifically, reflected light beam 6 is generated via reflection from the interface 7 between the atmospheric air and the charge transport layer 4, reflected light beam 8A is generated via reflection from the interface 9A between the charge transport layer 4 and the charger generating layer 3, reflected light beam 8B is generated via reflection from the interface 9B between the charge generating layer 3 and the undercoat layer 2, and reflected light beam 8C is generated via reflection from the interface 9C between the undercoat layer 2 and the substrate 1. The primary reflections that contribute to the interference-fringe print defect producing interference effect are the reflected beam 6 generated at the interface 7 between the surrounding atmospheric air and the charge transport layer 4 and the reflected beam 8C from the interface 9C between the undercoat layer 2 and the substrate 1, where the differences in optical indices are the greatest.
Methods have been proposed to suppress the charge transport layer/air interface specular reflection, including roughening of the charge transport layer surface by introducing micron size SiO.sub.2 dispersion and other particles into the charge transport layer, applying an appropriate overcoating layer and the like.
Methods have also been proposed to suppress the intensity of substrate surface specular reflection, e.g., coating specific materials such as anti-reflection materials and light scattering materials on the substrate surface and roughening methods such as dry blasting and liquid honing of the substrate surface. However, such methods must achieve their primary objective of eliminating substrate surface reflections without adversely impacting the electrical parameters or print quality of photoreceptors into which they are incorporated.
Patents on interference-fringe effect suppression include U.S. Pat. No. 5,219,691 to Fukuda et al., U.S. Pat. No. 4,618,552 to Tanaka et al., U.S. Pat. No. 4,741,918 to Nagy de Nagybaczon et al., U.S. Pat. No. 4,904,557 to Kubo et al, U.S. Pat. No. 4,134,763 to Fujimura et. al., U.S. Pat. No. 5,096,792 to Simpson et al., and U.S. Pat. No. 5,051,328 to Andrews et al.
A typical liquid honing process, described for example in Rasmussen et al., U.S. Pat. No. 5,573,445, is a technique to create a highly scattered surface on a metallic substrate, and is used in some multilayered devices to eliminate the interference-fringe effect. A liquid honing process, however, is disadvantageous in certain situations because it may require a relatively thick undercoat layer in the photoreceptor and the liquid honing process is an extra step after diamond lathing which thereby increases the cost of production of a substrate.
The present invention renders optional an undercoat layer described in more detail below, thereby decreasing the photoreceptor fabrication cost. In the present invention, a metal oxide layer is not considered an undercoat layer because the metal oxide layer is an integral part of the substrate and it is not coated onto the substrate. In typical multilayered photoreceptors, a resin layer is required to be inserted as an undercoat layer between the substrate and the photosensitive layers in order to provide a charge blocking capability, mechanical adhesion strength between the substrate and the photosensitive layers, and improved cyclic stability. Each undercoat layer (also referred herein as an intermediate layer) may be any layer conventionally employed between the substrate and the photosensitive layer as illustrated for example in Tanaka et al., U.S. Pat. No. 4,618,552 and Andrews et al., U.S. Pat. No. 5,051,328, the disclosures of which are totally incorporated herein by reference. Accordingly, the undercoat or intermediate layer may be a subbing layer, barrier layer, adhesive layer, and the like. The intermediate layer may be formed of, for example, a mixture of acetyl acetone zirconium tributoxides, and gamma amino propyltriethoxy silane, casein, polyvinyl alcohol, nitrocellulose, ethyleneacrylic acid copolymer, polyamide (nylon 6, nylon 615, nylon 610, copolymerized nylon, alkoxymethylated nylon, and the like), polyurethane, gelatin, and the like. If needed, a separate adhesive layer may be added between the intermediate layer and the subsequently applied layers to improve adhesion. Typical adhesive layers include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrrolidone, polycarbonate, polyurethane, polymethyl methacrylate, and the like as well as mixtures thereof. The intermediate layer may be deposited by any conventional means such as dip coating, spray coating, and vapor deposition and preferably has a thickness of from about 0.05 to about 5 microns.
Typical undercoat resin layers, however, exhibit poor environmental cyclic stability due to the fact that the volume resistivity of a resin greatly depends on the ionic conductivity and is strongly affected by temperature and humidity conditions. Many proposals have been made to form an undercoat layer using organic metal compounds or silane coupling agents to improve upon the environmental effects. U.S. Pat. No. 5,252,422 to Okano et al. and U.S. Pat. No. 5,188,916 to Hodumi et al.,for example, discuss the use of organic metal chelate compounds or organic metal alkoxide compounds with silane coupling agents as an improved undercoat layer in a multilayered photoreceptor for visible light xerographic applications. When this type of an undercoat material is used in combination with a roughened substrate for interference fringe suppression for printer applications where a coherent exposure light source is used, an addition of a resin is required to increase the thickness of the undercoat layer to ensure continuous coverage to avoid charge injection from the substrate. Examples of a print defect caused by charge injection from the substrate include a cluster of black spots in white background in a discharge area development (DAD) system, which are commonly known as "pepper spots." Thick undercoat layers, however, produce undesirable electrical effects including a high residual voltage build up and poor cyclic stability.
Thus, there is a need, which the present invention addresses, for a new photoreceptor design that minimizes or suppresses the interference-fringe effect while maintaining good performance characteristics.